Evolution of an intermediate C 4 photosynthesis in the non-foliar tissues of the Poaceae.

Carbon concentrating mechanisms (CCMs) in plants are abaptive features that have evolved to sustain plant growth in unfavorable environments, especially at low atmospheric carbon levels and high temperatures. Uptake of CO 2 and its storage in the aerenchyma tissues of Lycopsids and diurnal acidity fluctuation in aquatic plants during the Palaeozoic era (ca. 300 Ma.) would represent the earliest evolution of a CCM. The CCM parts of the dark reactions of photosynthesis have evolved many times, while the light reactions are conserved across plant lineages. A C 4 type CCM, leaf C 4 photosynthesis is evolved in the PACMAD clade of the Poaceae family. The evolution of C 4 photosynthesis from C 3 photosynthesis was an abaptation. Photosynthesis in reproductive tissues of sorghum and maize (PACMAD clade) has been shown to be of a weaker C 4 type (high CO 2 compensation point, low carbon isotope discrimination, and lack of Rubisco compartmentalization, when compared to the normal C 4 types) than that in the leaves (normal C 4 type). However, this does not fit well with the character polarity concept from an evolutionary perspective. In a recent model proposed for CCM evolution, the development of a rudimentary CCM prior to the evolution of a more efficient CCM (features contrasting to a weaker C 4 type, leading to greater biomass production rate) has been suggested. An intermediate crassulacean acid metabolism (CAM) type of CCM (rudimentary) was reported in the genera, Brassia, Coryanthes, Eriopsis, Peristeria, of the orchids (well-known group of plants that display the CAM pathway). Similarly, we propose here the evolution of a rudimentary CCM (C 4 -like type pathway) in the non-foliar tissues of the Poaceae, prior to the evolution of the C 4 pathway as identified in the leaves of the C 4 species of the PACMAD clade.